Extreme ultraviolet light, e.g., electromagnetic radiation having a wavelength of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers. Here and elsewhere herein the term “light” is used even though it is understood that the radiation described using that term may not be in the visible part of the spectrum.
Methods for generating EUV light include converting a target material from a liquid state into a plasma state. The target material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV part of the spectrum. In one such method, often termed laser produced plasma (“LPP”), the required plasma is produced by using a laser beam to irradiate and so to vaporize a target material having the required line-emitting element to form a plasma in an irradiation region.
The target material may take many forms. It may be solid or a molten. If molten, it may be dispensed in several different ways such as in a continuous stream or as a stream of discrete droplets. As an example, the target material in the discussion which follows is molten tin which is dispensed as a stream of discrete droplets. It will be understood by one of ordinary skill in the art, however, that other target materials, phases of target materials, and delivery modes for target materials may be used.
The energetic radiation generated during de-excitation and recombination of ions in the plasma propagates from the plasma omnidirectionally. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror” or simply a “collector”) is positioned to collect, direct, and, in some arrangements, focus the light to an intermediate location. The collected light may then be relayed from the intermediate location to where it is to be used, for example, to a set of scanner optics and ultimately to a wafer in the case where the EUV radiation is to be used for semiconductor photolithography.
The target material is introduced into the irradiation region by a target material dispenser. The target material dispenser is supplied with target material in a liquid or solid form. If supplied with target material in a solid form the target material dispenser melts the target material. The target material dispenser then dispenses the molten target material as a series of droplets into the vacuum chamber containing the irradiation region.
As can be appreciated, one technical requirement for implementation of a target material dispenser is introduction of liquid target material into the area where it will be irradiated. This requires some form of nozzle or jetting structure. One solution for the nozzle uses a glass capillary. A drawback of using a glass capillary is that it is not compatible with the pressures (6000 to 8000 psi) that the nozzle is preferably able to withstand. Also, the nozzle is preferably configured to permit stable modulation of the droplet stream. One method of modulating the droplet stream is to use a tube with a piezoelectric element. When a glass capillary is used as the nozzle it is affixed to the piezoelectric tube, for example, using an epoxy. This arrangement may exhibit instabilities over time. Also, a glass capillary is susceptible to malfunction, either in the form of missing droplets or misdirected droplets, due to particle contamination. There thus remains a need to provide a nozzle structure that can operate reliably under the required operating conditions, including high pressure, that are needed to dispense liquid target material, and to be able to do so stably over time and despite particle contamination.